You're pulling the trigger too slowly. Try a quick, crisp jerk and equally quick release. If you squeeze and hold the trigger, another round will go off as you release.
Wow, totally wrong on the whole technique and the worst you can do for accuracy.
BTW, I to have MAK90 and I both slowly squeeze the trigger and hold it back.
The only time it will fire again (sans a screwed up fire control group) is if you let go (it bounces) and then pull the thing again, period.
If you squeeze and hold firmly it will never fire a second time, it simply cannot.
It can only fire if you get into an oscillation of fire, recoil as a result of loose hold, the the trigger bounces back to the fire position and it gets pulled again which is what this sounds like.
A good overall grip, squezze even after it fire adn hold bakc and no sisue.